1. The Field of the Invention
This invention relates to sidepocket mandrels for use in underground wells and, more particularly, to novel apparatus and methods for providing a lightweight, sidepocket mandrel formed largely of a drillable, non-metal material.
2. The Background Art
Underground wells are typically constructed so as to include a large pipe called the "casing" which extends into the earth the entire length of the well. The casing helps ensure the mechanical integrity of the well so as, for example, to keep oil and gas from contaminating adjacent water supplies. Perforations are provided through the casing at the levels of various zones in the earth containing oil or gas. Packers are also provided within the casing to isolate these zones from the upper portion of the well and also to isolate adjacent zones from each other.
A typical well structure also comprises a smaller pipe called the "tubing." The tubing is installed within the casing so as to pass through the packers and also extend the entire length of the well. In oil or gas producing wells, it is typically the tubing which conveys the oil or gas to the wellhead. Similarly, the tubing is commonly used to convey injection material into injection wells.
In order to permit communication between the tubing and the casing, sidepocket mandrels are installed as part of the tubing string. Each mandrel typically has a port which communicates with the casing, and an adjacent valve receiving pocket is provided for installing a valve mechanism. Such valves thus allow for controlled communication between the tubing and the casing annulus surrounding the tubing at the various zone levels at which the sidepocket mandrels are installed. Thus, for example, a sidepocket mandrel may be provided with an orifice injection valve for the injection of water, corrosion inhibitors or other materials into the various zones of injection wells. Similarly, gas lift valves may be installed in the sidepocket mandrels in producing wells so as to carry out conventional gas lift methods.
A wireline running tool is usually employed to install a valve in a sidepocket mandrel. The running tool attached to the wireline is used to lower the valve into a sidepocket mandrel, and a latch mechanism is provided on the valve to lock the valve into place in the mandrel.
When it is subsequently desired to remove a valve from a sidepocket mandrel, a wireline is again run into the well to the level of the valve. A pulling tool attached to the end of the wireline is then used to secure the end of the valve latch. An upward jarring on the wireline then shears a pin in the latch mechanism, thereby releasing the valve from the sidepocket mandrel and allowing it to be removed by the wireline from the well.
Typically, sidepocket mandrels are formed of a metal alloy. Thus, for example, the mandrel's valve receiving pocket is often machined separately and then welded into the side of a tube to form the sidepocket mandrel. Alternatively, the lower portion of the mandrel including the valve receiving pocket may be machined as a single unit and then welded to a tube forming the upper portion of the mandrel.
There are several significant disadvantages with this prior art mandrel construction. For example, the need to weld separate metal components together weakens the mandrel body. Frequently, the mandrel body will begin to leak along the weld seams, thereby interfering with the operation of the mandrel.
One of the most significant problems associated with the prior art mandrels is their susceptibility to corrosion. As a result of corrosion, holes may be formed in the mandrel which make the mandrel unable to function effectively in the well. Consequently, the well installation eventually fails.
When a well installation fails, as described above, it is necessary to remove the defective parts from the well structure. However, due to scale (iron sulfide) build up and the accumulation of other minerals on metal parts, the mandrel and other well components typically cannot be retrieved at once. This necessitates multiple fishing operations which can be very time consuming and expensive.
In an attempt to avoid the significant time and expense of retrieving components from a failed well, one may simply try to destroy the failed well components in place. The use of metal components, however, renders even this procedure somewhat time consuming and expensive. Moreover, the drilling of metal components often damages the casing of the well which may render the well entirely useless.